SQL查询中如何对一个字段判断后再查询其他表？
因为要导出一些数据，现在出现这种情况：ffvv2.DESCRIPTION 线路, hv.lineway_name 需求申请线路,这两个字段中抓取的是不同的表，如何设计SQL代码实现比如在ffvv2.DESCRIPTION为空的时候，去查找 hv.lineway_name的信息，并且在导出的数据中是放在同一个字段中；不需要分开两个字段。
select distinct msib.segment1 编码,
msib.description 描述,
flv.MEANING 物资类型,
mtt.transaction_type_name 事务处理类型,
mmt.attribute2 as 领料单号 ,
cm.workorder_type 工单类型,
cm.comments 描述,
va.DESCRIPTION 部门,
ffvv2.DESCRIPTION 线路,
hv.lineway_name 需求申请线路,
mmt.transaction_date 事务处理时间,
fu.user_name 操作人,
mmt.subinventory_code 仓库,
from apps.mtl_material_transactions mmt,
apps.mtl_system_items_b msib,
apps.FND_FLEX_VALUES_VL va ,
apps.mtl_transaction_lot_numbers mtln,
apps.fnd_lookup_values_vl flv,
apps.fnd_user fu,
inv.mtl_transaction_types mtt,
apps.mtl_secondary_inventories msi,
apps.ORG_ORGANIZATION_DEFINITIONS ou,
apps.cux_inv_mo_headers cm,-----工单
apps.fnd_flex_values_vl ffvv2,----线路
apps.pa_tasks pt,----项目号
apps.cux_inv_demand_headers_v hv-----需求申请视图
where mmt.inventory_item_id = msib.inventory_item_id
and mmt.attribute2 = cm.mo_code(+)
AND ffvv2.flex_value_id(+) = cm.lineway_id
AND cm.task_id = pt.task_id(+)
and mmt.organization_id = ou.organization_id
and mtln.transaction_id(+) = mmt.transaction_id
and msib.organization_id = mmt.organization_id
and mmt.organization_id = msi.organization_id
and mmt.transaction_type_id = mtt.transaction_type_id
and mmt.subinventory_code = msi.secondary_inventory_name
and fu.user_id = mmt.created_by
and va.FLEX_VALUE_ID(+) = mmt.attribute6
and msib.item_type = flv.LOOKUP_CODE
and flv.LOOKUP_TYPE = 'ITEM_TYPE'
and mmt.attribute2=hv.demand_code(+)
and mmt.creation_date >=
to_date('2015-01-01 00:00:00', 'yyyy-mm-dd hh24:mi:ss')
and mmt.creation_date <
to_date('2015-03-01 00:00:00', 'yyyy-mm-dd hh24:mi:ss')
and msib.organization_id in
(124, 140, 1923, 841, 861, 842, 1144, 139)
------解决思路----------------------
select distinct msib.segment1 编码,
msib.description 描述,
flv.MEANING 物资类型,
mtt.transaction_type_name 事务处理类型,
mmt.attribute2 as 领料单号 ,
cm.workorder_type 工单类型,
cm.comments 描述,
va.DESCRIPTION 部门,
nvl(ffvv2.DESCRIPTION,hv.lineway_name) 线路,
hv.lineway_name 需求申请线路,
mmt.transaction_date 事务处理时间,
fu.user_name 操作人,
mmt.subinventory_code 仓库,
from apps.mtl_material_transactions mmt,
apps.mtl_system_items_b msib,
apps.FND_FLEX_VALUES_VL va ,
apps.mtl_transaction_lot_numbers mtln,
apps.fnd_lookup_values_vl flv,
apps.fnd_user fu,
inv.mtl_transaction_types mtt,
apps.mtl_secondary_inventories msi,
apps.ORG_ORGANIZATION_DEFINITIONS ou,
apps.cux_inv_mo_headers cm,-----工单
apps.fnd_flex_values_vl ffvv2,----线路
apps.pa_tasks pt,----项目号
apps.cux_inv_demand_headers_v hv-----需求申请视图
where mmt.inventory_item_id = msib.inventory_item_id
and mmt.attribute2 = cm.mo_code(+)
AND ffvv2.flex_value_id(+) = cm.lineway_id
AND cm.task_id = pt.task_id(+)
and mmt.organization_id = ou.organization_id
and mtln.transaction_id(+) = mmt.transaction_id
and msib.organization_id = mmt.organization_id
and mmt.organization_id = msi.organization_id
and mmt.transaction_type_id = mtt.transaction_type_id
and mmt.subinventory_code = msi.secondary_inventory_name
and fu.user_id = mmt.created_by
and va.FLEX_VALUE_ID(+) = mmt.attribute6
and msib.item_type = flv.LOOKUP_CODE
and flv.LOOKUP_TYPE = 'ITEM_TYPE'
and mmt.attribute2=hv.demand_code(+)
and mmt.creation_date >=
to_date('2015-01-01 00:00:00', 'yyyy-mm-dd hh24:mi:ss')
and mmt.creation_date <
to_date('2015-03-01 00:00:00', 'yyyy-mm-dd hh24:mi:ss')
and msib.organization_id in
(124, 140, 1923, 841, 861, 842, 1144, 139)
------解决思路----------------------
同意一楼，除了nvl还可以用coalesce
------解决思路----------------------
NVL(expr1,expr2)楼主你值得拥有！